The present invention relates to a conductivity detector for use in chromatography systems.
Prior art conductivity detectors are utilized with chromatography systems in which a chemical solution passing through a chromatographic column to a liquid conduit is connected to a conductivity cell. Fluctuations in ionic concentration in the conductivity cell produce an electrical signal proportional to the amount of ionic material, which is detected by a conductivity meter. The detected electrical signal is in turn generally connected to a display means such as a chart recorder which provides a visible display to a user.
Prior art conductivity detectors have in general been deficient for use as laboratory grade instruments because of poor absolute accuracy and poor linearity. Because conductivity cells do not have a constant current vs. conductance relationship, prior art detectors have not in general been able to compensate for this non-linearity. In addition, prior art detectors are not capable of completely nulling out non-linear capacitive components of the chemical solution to be measured. In order to reduce the effects of solution temperature variation on a detector's baseline, it would be desirble for a conductivity detector to correct a measured conductivity to what conductivity would be at some fixed temperature (say 25.degree. C.) so that output baseline is independent of changes in solution temperature.
Prior art detectors attempt to correct for temperature changes using thermistors, but are limited such that it is sometimes worse than no correction at all. This is because thermistor temperature is mainly affected by the cell body temperature due to poor thermistor positioning, the thermistor response is relatively slow (typically greater than thirty seconds) and the detector correction circuitry does not follow correct exponential relationships. Prior art detectors typically use a thermistor in a feedback loop of an operational amplifier so that the correction only approaches an exponential over a very limited temperature range.
In view of the above background, it is an objective of the present invention to provide an improved conductivity detector for use in chromatography systems.